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NBC 7's Greg Bledsoe reports a few new details surrounding the deaths of four Marines at Camp Pendleton. A base official confirmed the incident happened in the Zulu impact area. (Published Wednesday, Nov. 13, 2013)

Four U.S. Marines were killed at Camp Pendleton in San Diego County Wednesday morning during a range maintenance operation, base officials confirmed.

According to Camp Pendleton officials, the deadly incident happened around 11 a.m. At this point, few details have been released about what actually occurred.

NBC News reports that a U.S. military official said the incident involved the detonation of some unexploded ordnance on a range at the base. The Marines involved in the accident were Explosive Ordnance Disposal Marines, NBC News reported.

NBC News sources inside the Pentagon said the Marines were not clearing the range, but instead doing some sort of training when the deadly detonation occurred.

An official from the public affairs office told NBC 7 San Diego that the incident happened in the Zulu impact area, which is located roughly at the center of the base. The impact area is where Marines fire artillery and drop bombs from aircraft.

The official said there was artillery training on base Wednesday, but not in that specific area. He said there was no firing going on at the Zulu impact area at the time of the fatal incident.

The official also clarified what is meant by the term “range maintenance,” saying it’s a fairly broad term used to describe a variety of upkeep on those firing ranges and can include anything from mowing the grass to actually clearing artillery.

Officials said they are now in the process of notifying the families of the Marines killed on base. The names of the deceased Marines will be released once their families are notified.

"We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident. Our first priority is to provide the families with the support they need during this difficult time."

The cause of the incident is under investigation.

This isn’t the first deadly training incident involving Camp Pendleton-based U.S. Marines.

This past February, a Camp Pendleton-based Marine died in an active-duty training exercise in Riverside County. The Marine was skydiving as part of training and went down near the Perris Valley Airport.